Re: PET schematics bugs

From: Andre Fachat (a.fachat_at_physik.tu-chemnitz.de)
Date: 1998-05-08 08:52:28

More on the PET I/O area size:

The 8032 schematics have the ROM for $E*** labeled as a 2316, meaning
it being 2k. But they have BA11 still connected, and it is connected
to the /CS line as in the 2001N, so it is only selected when
/SELE is active ($E***) and A11 is low -> 2k ROM ($E000-$E7FF), no mirror.

However, the 8032 PET mask /SELE for the ROM with x8xx, which is
active at addresses $*8**. This means the /SELE is _not_ active 
in $E8**.

So despite the labeling it should be possible to use a 4k ROM 2332,
which then has A11 at the rigth place and is selected at $E*** except 
$E8**, et voila, 256 byte I/O space.

So for 2k ROMs it should look like open-address space in $E900-$EFFF
and for 4k ROMs there is ROM in $E900-$EFFF - no further hardware
changes needed!

This should also be an easy way to put in almost 2k of own code 
into the ROM, without using $9*** and $A***.

Only you should not use a 4k ROM with the SuperPET, as it uses the
$EFE0-$EFFF as I/O space and you then cannot read from there anymore
(bus conflict).

Of course all this comes without warranty.

so long
Andre

Andre Fachat wrote:
> 
> One more comment to the 2001N schematics. One might wonder why the 
> ROM for $E*** is selected with /SELE only while the I/O is selected
> with /SELE and x8xx. This would suggest that in $E8** both would be
> selected and drive the bus, which would fail. 
> But: The 2316 has a low-active select line at pin 18, where the 
> 2332 has A11. But only a A11 is written in the schematics, so 
> you have to guess that the IC (if and only if 2k) is selected only
> when A11 is low, i.e. $E000-$E7FF.
> And you should better not put a 4k ROM in there :-)
> 

-- 
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